Semiconductor integrated circuit device and method of fabricating the same

ABSTRACT

To suppress oxidation of inner walls of element isolation grooves otherwise occurring during thermal oxidation processes. A nitrogen introducing layer less in diffusion coefficient relative to an oxidizing agent is formed at the surface portion of a silicon oxide film as buried within an element isolation groove. This nitrogen introduce layer functions as a barrier layer for precluding the oxidizer (such as oxygen, water or else) in vapor phase from diffusing into the silicon oxide film at thermal processing steps. The nitrogen introduce layer is formed by performing nitrogen ion implantation into an entire surface of a substrate and subsequently applying thermal processing to the substrate to thereby activate the nitrogen doped.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates generally to semiconductor integrated circuit devices and fabrication technologies thereof and, more particularly to useful techniques for application to semiconductor integrated circuit devices having circuit element isolation grooves with a insulative film buried within a groove as formed in a semiconductor substrate.

[0003] 2. Description of the Related Art

[0004] With an increase in miniaturization or shrinkage of semiconductor devices for use as on-chip circuit elements, a new element isolation technique is employed in lieu of prior known local oxidation of silicon (LOCOS) methods. This element isolation technique is the one that embeds or buries a silicon oxide film within a groove as formed in a semiconductor substrate to thereby form more than one element isolation groove, and is also known as the shallow trench isolation (STI) process.

[0005] To form the element isolation groove, the process starts with a step of applying thermal processing to the semiconductor substrate (simply referred to as “substrate” hereinafter) for fabrication of a thin silicon oxide film (also called pad oxide film) on or above its surface. This pad oxide film is formed for purposes of relaxation of stresses being applied to the substrate during baking for densification of the oxide silicon film buried within the groove at a later step and also protection of active regions during removal of a silicon nitride film that is used as a mask for oxidation.

[0006] Next, the process has a step to deposit a silicon nitride film at upper part of the pad oxide film by chemical vapor deposition (CVD) techniques and then remove portions of the silicon nitride film which reside in element isolation regions by etching with a patterned photoresist film as a mask. This silicon nitride film is used as a mask during formation of a groove(s) through etching of the substrate along with a protective film for protection of its underlying substrate surface against oxidation.

[0007] Next, after having formed a groove in the substrate by dry etching with the silicon nitride film as a mask, the substrate is thermally oxidized to thereby fabricate a thin silicon oxide film on inner walls of the groove. This silicon oxide film is formed for purposes of removal of etch damages occurring at the groove inner walls and also stress relaxation of a silicon oxide film to be buried within the groove at a later step.

[0008] Next, after having deposited by CVD method a silicon oxide film over the substrate to bury it within the groove, the substrate is thermally processed to thereby density this silicon oxide film. This densification is for improvements in film quality of the silicon oxide film as buried within the groove.

[0009] Next, the process has a step to remove the silicon oxide film overlying the silicon nitride film by chemical-mechanical polishing (CMP) techniques causing the silicon oxide film to remain within the groove, thus completing more than one element isolation groove. Thereafter, the silicon nitride film that has been used as the mask during formation of the groove in the substrate is removed by wet etch techniques, thus forming in active regions semiconductor circuit elements such as metal insulator semiconductor field effect transistors (MISFETs).

[0010] Unfortunately, it has been pointed out that the element isolation groove as formed by the above-discussed methodology suffers from degradation in device characteristics. This can occur due to a mechanism which follows. At several thermal oxidation process steps during fabrication of semiconductor elements in active regions, oxidizing agents or oxidizers such as oxygen and water components in an atmosphere behave to invade the inside of an element isolation groove and then oxidize the inner walls thereof, resulting in formation of a silicon oxide film. Due to thermal volume expansion of such silicon oxide film, the active region is applied compressive stress force. This stress application can result in creation of unwanted crystal defects and/or dislocations at a portion of the substrate in the active region, causing the device characteristics to degrade. In view of the fact that the compressive stress being applied to the active region by oxidation of the isolation groove inner walls increases with a decrease in isolation groove width as resulted from a decrease in feature size of on-chip semiconductor devices, this problem is a serious bar to successful achievement of miniaturization or shrinkage of the semiconductor devices.

[0011] Another problem faced with the above-stated element isolation groove as has been reported until today is that a drain current can rush to flow even upon application of a low gate voltage. This is called the “kink” or “hump” characteristics among experts in the semiconductor device art. This problem occurs due to a mechanism which follows. The silicon oxide film buried in a groove is etched away through more than two hydrofluoric-acid cleaning process steps to be done during formation of a semiconductor circuit element in an active region, resulting in a portion of the silicon oxide film at an element isolation groove edge being retrograded or “recessed” downwardly, which in turn causes a gate insulating film as formed over a substrate surface portion of the active region to locally decrease in thickness. This local thinning of such gate insulating film leads to undesired drain current flow even upon application of a low gate voltage.

[0012] One prior known approach to suppressing oxidation in the inner walls of element isolation grooves is to fabricate a thin silicon nitride film along the isolation groove inner walls in a way as taught, for example, from U.S. Pat. No. 5,447,884.

[0013] In addition, JP-A-2000-31267 discloses therein a remedy for formation of an unwanted groove at the outer periphery of an element isolation groove due to simultaneous etching of an oxidation-preventing silicon nitride film formed on isolation groove inner walls during removal by wet etching of a mask-use silicon nitride film as used for formation of grooves in a substrate, wherein the remedy employs a technique for increasing its etching rate by doping through ion implantation a chosen impurity into the silicon nitride mask to thereby damage it.

[0014] JP-A-2000-306990 discloses therein a remedy for preventing the silicon nitride film within an element isolation groove from being recessed due to unintentional etching during removal by wet etching of a pad oxide film as has been formed beneath the mask-use silicon nitride film, wherein the remedy makes use of a technique for applying thermal processing to the silicon oxide film within the isolation groove in a nitride gaseous atmosphere after having etched away the silicon nitride mask and yet prior to removal of the pad oxide film.

[0015] Considering from a different viewpoint from the problem stated above, JP-A-8-227938 discloses a remedy for precluding undesired etching of the silicon oxide film within an element isolation groove as a result of overlapping of a portion of a through-going hole with the isolation groove due to possible mask positioning/alignment deviation during formation of the through-hole deep enough to reach its underlying substrate by etching of a silicon oxide film as deposited over the substrate, by use of a technique for forming a silicon nitride film overlying the silicon oxide film within the isolation groove to thereby use this silicon nitride film as an etch stopper during formation of the through-hole.

SUMMARY OF THE INVENTION

[0016] In recent years, in the manufacture of a variety of types of metal oxide semiconductor (MOS) devices including but not limited to complementary MOS (CMOS) devices with a plurality of types of MISFETs different in gate insulating film thickness from one another being formed on or over the same substrate, an increased number of hydrofluoric-acid cleaning and thermal oxidation process steps are required, which are to be effectuated at a stage between element isolation groove formation and fabrication of MISFETs. Due to this, with the related art methods stated supra, it becomes difficult to effectively suppress element isolation groove inner wall oxidation and/or silicon oxide film's retrogradation (recessing) at element isolation groove terminate end/edge portions, which would result in a decrease in reliability and mass-production yields of MOS devices.

[0017] It is therefore a primary object of the present invention to provide a technique for effectively suppressing oxidation on inner walls of circuit element isolation grooves otherwise occurring during thermal oxidation processes.

[0018] It is another object of this invention to provide a technique for successfully suppressing retrogradation (recessing) of element isolation groove end portions otherwise occurring during hydrofluoric acid cleaning processes.

[0019] These and other objects, features and advantages of the invention will be apparent from the following more particular description of preferred embodiments of the invention, as illustrated in the accompanying drawings.

[0020] A brief description of some representative ones of the inventive principles as disclosed and claimed herein is as follows.

[0021] A semiconductor integrated circuit device of the present invention is the one that comprises a semiconductor substrate having at its principal surface a first area and a second area planarly surrounded by the first area, a first insulative film formed over the principal surface of the semiconductor substrate in the second area, a conductor piece formed over the first insulative film, a semiconductor region formed at the semiconductor substrate principal surface on both sides of the conductor piece, and an oxide film being formed at the semiconductor substrate principal surface in the first area and having a film thickness greater than that of the first insulative film, wherein nitrogen is introduced at a surface portion of the oxide film.

[0022] A method of fabricating a semiconductor integrated circuit device fabrication method of the instant invention includes the steps which follows:

[0023] (a) exposing a first area of a semiconductor substrate principal surface to thereby selectively form a first insulative film at the semiconductor substrate principal surface in a second area as surrounded by the first area;

[0024] (b) forming a groove in the semiconductor substrate surface in the first area;

[0025] (c) forming a second insulative film within the groove and over the first insulative film in such a way as to bury the groove;

[0026] (d) applying polishing to the second insulative film to thereby permit the second insulative film to selectively reside within the groove;

[0027] (e) introducing nitrogen into the second insulative film within the groove;

[0028] (f) removing the first insulative film of the second area;

[0029] (g) forming a third insulative film at the semiconductor substrate surface in the second area;

[0030] (h) forming a conductor piece over the third insulative film; and

[0031] (i) introducing in the second area an impurity into the semiconductor substrate surface on both sides of the conductor piece.

BRIEF DESCRIPTION OF THE DRAWINGS

[0032]FIG. 1 is a diagram illustrating, in cross-section, main part of a semiconductor substrate for indication of a method of fabricating a semiconductor integrated circuit device in accordance with one embodiment of the present invention.

[0033]FIG. 2 is a diagram depicting in cross-section main part of the semiconductor substrate for indication of the semiconductor integrated circuit device fabrication method in accordance with the embodiment of this invention.

[0034]FIG. 3 is a diagram showing in cross-section main part of the semiconductor substrate for indication of the semiconductor integrated circuit device fabrication method in accordance with the embodiment of the invention.

[0035]FIG. 4 is a cross-sectional diagram of main part of the semiconductor substrate for indication of the semiconductor integrated circuit device fabrication method in accordance with the embodiment of the invention.

[0036]FIG. 5 is a sectional diagram of main part of the semiconductor substrate for indication of the semiconductor integrated circuit device fabrication method in accordance with the embodiment of the invention.

[0037]FIG. 6 is a sectional diagram of main part of the semiconductor substrate for indication of the semiconductor integrated circuit device fabrication method in accordance with the embodiment of the invention.

[0038]FIG. 7 is a sectional diagram of main part of the semiconductor substrate for indication of the semiconductor integrated circuit device fabrication method in accordance with the embodiment of the invention.

[0039]FIG. 8 is a sectional diagram of main part of the semiconductor substrate for indication of the semiconductor integrated circuit device fabrication method in accordance with the embodiment of the invention.

[0040]FIG. 9 is an enlarged sectional diagram of main part of the semiconductor substrate for indication of the semiconductor integrated circuit device fabrication method in accordance with the embodiment of the invention.

[0041]FIG. 10 is a sectional diagram of main part of the semiconductor substrate for indication of the semiconductor integrated circuit device fabrication method in accordance with the embodiment of the invention.

[0042]FIG. 11 is an enlarged sectional diagram of main part of the semiconductor substrate for indication of the semiconductor integrated circuit device fabrication method in accordance with the embodiment of the invention.

[0043]FIG. 12 is an enlarged sectional diagram of main part of the semiconductor substrate for indication of the semiconductor integrated circuit device fabrication method in accordance with the embodiment of the invention.

[0044]FIG. 13 is an enlarged sectional diagram of main part of the semiconductor substrate for indication of the semiconductor integrated circuit device fabrication method in accordance with the embodiment of the invention.

[0045]FIG. 14 is a sectional diagram of main part of the semiconductor substrate for indication of the semiconductor integrated circuit device fabrication method in accordance with the embodiment of the invention.

[0046]FIG. 15 is a sectional diagram of main part of the semiconductor substrate for indication of the semiconductor integrated circuit device fabrication method in accordance with the embodiment of the invention.

[0047]FIG. 16 is a sectional diagram of main part of the semiconductor substrate for indication of the semiconductor integrated circuit device fabrication method in accordance with the embodiment of the invention.

[0048]FIG. 17 is a sectional diagram of main part of the semiconductor substrate for indication of the semiconductor integrated circuit device fabrication method in accordance with the embodiment of the invention.

[0049]FIG. 18 is a sectional diagram of main part of the semiconductor substrate for indication of the semiconductor integrated circuit device fabrication method in accordance with the embodiment of the invention.

[0050]FIG. 19 is a sectional diagram of main part of the semiconductor substrate for indication of the semiconductor integrated circuit device fabrication method in accordance with the embodiment of the invention.

[0051]FIG. 20 is a sectional diagram of main part of the semiconductor substrate for indication of the semiconductor integrated circuit device fabrication method in accordance with the embodiment of the invention.

[0052]FIG. 21 is a sectional diagram of main part of the semiconductor substrate for indication of the semiconductor integrated circuit device fabrication method in accordance with the embodiment of the invention.

[0053]FIG. 22 is a sectional diagram of main part of the semiconductor substrate for indication of the semiconductor integrated circuit device fabrication method in accordance with the embodiment of the invention.

[0054]FIG. 23 is a sectional diagram of main part of the semiconductor substrate for indication of the semiconductor integrated circuit device fabrication method in accordance with the embodiment of the invention.

[0055]FIG. 24 is a sectional diagram of main part of the semiconductor substrate for indication of the semiconductor integrated circuit device fabrication method in accordance with the embodiment of the invention.

[0056]FIG. 25 is a sectional diagram of main part of the semiconductor substrate for indication of the semiconductor integrated circuit device fabrication method in accordance with the embodiment of the invention.

[0057]FIG. 26 is a diagram illustrating in cross-section main part of a semiconductor substrate for indication of a semiconductor integrated circuit device fabrication method in accordance with another embodiment of the present invention.

[0058]FIG. 27 is an enlarged diagram showing in cross-section main part of the semiconductor substrate for indication of the semiconductor integrated circuit device fabrication method in accordance with the another embodiment of the invention.

[0059]FIG. 28 is an enlarged sectional diagram of main part of the semiconductor substrate for indication of the semiconductor integrated circuit device fabrication method in accordance with the another embodiment of the invention.

[0060]FIG. 29 is an enlarged sectional diagram of main part of the semiconductor substrate for indication of the semiconductor integrated circuit device fabrication method in accordance with the another embodiment of the invention.

[0061]FIG. 30 is an enlarged sectional diagram of main part of the semiconductor substrate for indication of the semiconductor integrated circuit device fabrication method in accordance with the another embodiment of the invention.

[0062]FIG. 31 is a diagram showing in cross-section main part of a semiconductor substrate for indication of a semiconductor integrated circuit device fabrication method in accordance with still another embodiment of the invention.

[0063]FIG. 32 is a sectional diagram of main part of the semiconductor substrate for indication of the semiconductor integrated circuit device fabrication method in accordance with the still another embodiment of the invention.

[0064]FIG. 33 is a sectional diagram of main part of the semiconductor substrate for indication of the semiconductor integrated circuit device fabrication method in accordance with the still another embodiment of the invention.

[0065]FIG. 34 is a diagram showing in cross-section main part of a semiconductor substrate for indication of a semiconductor integrated circuit device fabrication method in accordance with yet another embodiment of the invention.

[0066]FIG. 35 is an enlarged sectional diagram of main part of the semiconductor substrate for indication of the semiconductor integrated circuit device fabrication method in accordance with the yet another embodiment of the invention.

[0067]FIG. 36 is an enlarged sectional diagram of main part of the semiconductor substrate for indication of the semiconductor integrated circuit device fabrication method in accordance with the yet another embodiment of the invention.

[0068]FIG. 37 is a diagram showing in cross-section main part of a semiconductor substrate for indication of a semiconductor integrated circuit device fabrication method in accordance with a further embodiment of the instant invention.

[0069]FIG. 38 is a sectional diagram of main part of the semiconductor substrate for indication of the semiconductor integrated circuit device fabrication method in accordance with the further embodiment of the invention.

[0070]FIG. 39 is a sectional diagram of main part of the semiconductor substrate for indication of the semiconductor integrated circuit device fabrication method in accordance with the further embodiment of the invention.

[0071]FIG. 40 is a sectional diagram of main part of the semiconductor substrate for indication of the semiconductor integrated circuit device fabrication method in accordance with the further embodiment of the invention.

[0072]FIG. 41 is a sectional diagram of main part of the semiconductor substrate for indication of the semiconductor integrated circuit device fabrication method in accordance with the further embodiment of the invention.

[0073]FIG. 42 is a diagram showing in cross-section main part of a semiconductor substrate for indication of a semiconductor integrated circuit device fabrication method in accordance with another further embodiment of the invention.

[0074]FIG. 43 is a sectional diagram of main part of the semiconductor substrate for indication of the semiconductor integrated circuit device fabrication method in accordance with the another further embodiment of the invention.

[0075]FIG. 44 is a sectional diagram of main part of the semiconductor substrate for indication of the semiconductor integrated circuit device fabrication method in accordance with the another further embodiment of the invention.

[0076]FIG. 45 is a sectional diagram of main part of the semiconductor substrate for indication of the semiconductor integrated circuit device fabrication method in accordance with the another further embodiment of the invention.

[0077]FIG. 46 is a sectional diagram of main part of the semiconductor substrate for indication of the semiconductor integrated circuit device fabrication method in accordance with the another further embodiment of the invention.

[0078]FIG. 47 is a sectional diagram of main part of the semiconductor substrate for indication of the semiconductor integrated circuit device fabrication method in accordance with the another further embodiment of the invention.

[0079]FIG. 48 is a sectional diagram of main part of the semiconductor substrate for indication of the semiconductor integrated circuit device fabrication method in accordance with the another further embodiment of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0080] Several preferred embodiments of the present invention will now be described in detail with reference to the accompanying drawings below. Note that in all drawings for explanation of the embodiments, those having the same functions are designated by the same reference characters with any repetitive explanations thereof eliminated.

[0081] Embodiment 1:

[0082] A complementary MISFET (CMOSFET) fabrication method in accordance with a first embodiment of the invention will be explained by use of FIGS. 1 through 25.

[0083] Initially as shown in FIG. 1, a substrate 1 made of p-type single-crystalline silicon having resistivity of 1 to 10 Ω cm for example is subjected to thermal oxidation at approximately 850° C. to thereby fabricate on its surface a thin silicon oxide film (pad oxide film) 2 to a thickness of about 10 nm; thereafter to deposit by chemical vapor deposition (CVD) techniques a silicon nitride film 3 with a thickness of about 120 nm on the silicon oxide film 2.

[0084] The silicon oxide film 2 is formed for relaxation of any possible stress occurring at an interface between the substrate 1 and silicon nitride film 3 to thereby preclude creation of crystal defects such as dislocations or the like at the surface of substrate 1 due to this stress. The silicon nitride film 3 is for use as a mask during formation of more than one groove through selective etching of specified portions of the substrate 1 which reside in circuit element isolation regions. The silicon nitride film 3 is hardly oxidizable in nature and thus is used also as a protective film for prevention of any unwanted oxidation at the surface of its underlying substrate 1.

[0085] Next, as shown in FIG. 2, selected portions of the silicon nitride film 3 and silicon oxide film 2 in the element isolation regions is subject to dry etching process with a photoresist film 40 as formed on the silicon nitride film 3 in the active regions being used as a mask to thereby cause the substrate 1 to be exposed at its surface portions in the element isolation regions.

[0086] Next, after having removed the photoresist film 40, the silicon nitride film 3 is used as a mask to apply dry etching to the substrate 1 in the element isolation regions, thereby forming grooves 4 a to a depth of 350 nm as shown in FIG. 3.

[0087] Next, after completion of cleaning of the resultant exposed surface portions of the substrate 1 to inner walls of the grooves 4 a by use of SC-1 liquid (mixture solution of ammonia water and hydrogen peroxide water), SC-2 (mixture of hydrochloric acid and hydrogen peroxide water) and hydrofluoric acid (or alternatively hydrofluoric-acid/ammonia-water mixture), the substrate 1 is thermally oxidized at about 800 to 900° C. as shown in FIG. 4, thus forming on the inner walls of each groove 4 a a thin silicon oxide film 5 with a thickness of about 10 nm. The silicon oxide film 5 is formed for recovery of dry etch damages occurring at the inner walls of groove 4 a while at the same time lessening or “relaxing” stress occurring at an interface between the substrate 1 and a silicon oxide film 6 to be later buried within the individual groove 4 a.

[0088] Next, as shown in FIG. 5, CVD methods is used to deposit over the substrate 1 a silicon oxide film 6 to a thickness of about 450 to 500 nm. The silicon oxide film 6 is completely buried within each groove 4 a in such a way that this film 6 is deposited by step coverage-excellent film fabrication methods such as CVD techniques with oxygen and tetraethoxysilane ((C₂H₅)₄Si) used as source gases, by way of example.

[0089] Next, after having thermally processed the substrate 1 at about 900° C. and densified the silicon oxide film 6 to thereby make the film denser, the silicon oxide film 6 is dry-etched partway at its portions in the active regions with a photoresist film 41 that is formed at upper part of the grooves 4 a as a mask as shown in FIG. 6, thereby letting the silicon oxide film 6's surface height in the active regions to be substantially the same as that in the element isolation regions. It should be noted here that the densification of silicon oxide film 6 may alternatively be done after effectuation of this dry etch process. If this is the case, it is possible to shorten the length of a time period as taken for densification. This can be said because the silicon oxide film 6 is less in thickness than before dry etching.

[0090] Next as shown in FIG. 7, after the removal of the photoresist film 41, chemical mechanical polishing (CMP) techniques is used to polish the silicon oxide film 6 for planarization of its surface. This polishing is done while using as a stopper the silicon nitride film 3 that covers the substrate 1 in the active regions and is terminated when the surface of silicon nitride film 3 is exposed—that is, at a time point that the silicon nitride film 3's surface becomes coplanar with a surface of the silicon oxide film 6 within each groove 4 a.

[0091] Next as shown in FIG. 8 and FIG. 9 (diagram enlargedly showing main part of a structure shown in FIG. 8), the surface of silicon oxide film 6 within each groove 4 a is wet-etched using either hydrofluoric acid or hydrofluoric acid buffer solution (mixture liquid of hydrofluoric acid and ammonia water), thereby causing the surface of silicon oxide film 6 to slightly retrograde or regress downwardly. Although this wet etching is not any essential process, in case a level difference between the surface of silicon oxide film 6 and the surface of substrate 1 is relatively large, nitrogen to be doped by ion implantation into silicon oxide film 6 at the next step will possibly fail to reach a region deeper than the substrate 1's surface. Accordingly, in such a case, the above-noted wet etching is effectuated causing the silicon oxide film 6's surface to offer downward retrogradation to thereby lessen the level difference relative to the substrate 1's surface.

[0092] Next as shown in FIG. 10, nitrogen is ion-implanted into an entire surface of the substrate 1; subsequently, the substrate 1 is thermally processed to activate the nitrogen doped whereby a nitrogen introduction layer 7 is formed at a selected surface portion of the silicon oxide film 6. At this time it is desirable that a dose amount or “dosage” of nitrogen be set at 1×10¹⁵/cm² or greater.

[0093] During ion implantation of nitrogen into the substrate 1, the acceleration energy of nitrogen ions is controlled to ensure that a bottom portion of the nitrogen introduce layer 7 reaches a level lower than the surface of a portion of substrate 1 in an active region while at the same time letting a thickness of from the nitrogen introduce layer 7's surface to its bottom measure 10 nm or more.

[0094] Additionally, upon introduction of nitrogen ions into the surface of substrate 1 in the active region, an undesired energy level can be formed at an interface between the substrate 1 and the gate insulating film of a MISFET to be later fabricated, resulting in a decrease in breakdown voltage of the gate insulating film and/or a variation in threshold voltage of the MISFET. Accordingly, in the event that the substrate 1 is doped with nitrogen by ion implantation, the acceleration energy of nitrogen ions is appropriately controlled to prevent arrival of nitrogen ions at the substrate 1's surface after penetration of the silicon nitride film 3.

[0095] The nitrogen introduce layer 7 as formed at the surface portion of silicon oxide film 6 has a structure similar to the so-called silicon oxynitride (SiON) with part of Si—O coupling or bonding for constitution of the silicon oxide film 6 being replaced by Si—N bond, which is featured in that the diffusion coefficient relative to oxidizers such as oxygen and/or water is noticeably less. More specifically the nitrogen introduce layer 7 formed at the surface portion of silicon oxide film 6 is expected to function as a barrier layer for suppression of diffusion of an oxidizer in gas phase into the silicon oxide film 6 during thermal processing of the substrate 1 in an oxidizing atmosphere, thereby suppressing oxidation of inner walls of the grooves 4 a.

[0096] Another feature of the above-noted nitrogen introduce layer 7 partially containing therein Si—N bonds is that an etching rate relative to hydrofluoric acid is lower than that of silicon oxides. In other words, the nitrogen introduce layer 7 may also function as a barrier layer for suppression of over-etching of the surface of silicon oxide film 6 while the surface of substrate 1 undergoes wet cleaning by hydrofluoric acid or hydrofluoric acid buffer liquid, thereby suppressing recessing at element isolation groove end portions, wherein the recessing would pose a problem during the wet cleaning process.

[0097] Next as shown in FIG. 12, using hot phosphoric acid the silicon nitride film 3 that covers the portions of the substrate 1 in the active regions is removed. With the processes up to here, circuit element isolation grooves 4 are formed in the substrate 1 at its portions in element isolation regions. Note here that since the aforesaid nitrogen introduce layer 7 that was formed at the surface portion of silicon oxide film 6 as buried within each element isolation groove 4 is such that a peak concentration point of the nitrogen doped by ion implantation is present at a deeper level than the surface, a ratio of Si—O bond is relatively high at locations adjacent to its surface whereas a ratio of Si—N bonds is relatively high in the inside thereof. Due to this, the nitrogen introduce layer 7 is little etched during etching of the silicon nitride film 3 by hot phosphoric acid; thus, as enlargedly shown in FIG. 13, a stair-step like surface difference substantially equivalent to the thickness of silicon nitride film 3 takes place between a surface portion of substrate 1 in the individual active region and the surface of nitrogen introduce layer 7 immediately after removal of the silicon nitride film 3.

[0098] Next, after having removed portions of the silicon oxide film 2 exposed to the substrate 1's surface in the active regions by use of hydrofluoric acid-based etching liquid (hydrofluoric acid or hydrofluoric acid buffer solution), the substrate 1 is thermally oxidized at about 850° C. to thereby form a thin silicon oxide film 10 with a thickness of about 10 nm on the surface portions of substrate 1 in the active regions as shown in FIG. 14. The silicon oxide film 10 is formed for reduction of damages of substrate 1 otherwise occurring due to impurity ion implantation as will be done at the next process step.

[0099] The nitrogen introduce layer 7 less in diffusion coefficient relative to oxidizer is formed at a surface portion of the silicon oxide film 6 as buried within the individual element isolation groove 4. With such an arrangement, the oxidizer in gas phase (oxygen, water and the like) is prevented from diffusing into the silicon oxide film 6 at thermal processing step for fabrication of the above-stated silicon oxide film 10, which in turn makes it possible to suppress oxidation of the inner walls of element isolation groove 4.

[0100] Note that although upon removal of the silicon oxide film 2 on the surface portions of substrate 1 in active regions by use of hydrofluoric acid-based etchant the nitrogen introduce layer 7 partially containing Si—O bonds is also little etched resulting in a decrease in step-like surface difference between a surface portion of substrate 1 in an active region and the surface of nitrogen introduce layer 7, the nitrogen introduce layer 7 which is lower than silicon oxides in etching rate relative to hydrofluoric acid will hardly be overetched, thereby enabling suppression of defects as to downward retrogradation (recessing) of the silicon oxide film 6 at edges of the element isolation grooves 4.

[0101] Next as shown in FIG. 15, boron is doped by ion implantation into surface portions of the substrate 1 in specified areas for fabrication of n-channel type MISFETs while covering or masking the remaining surface portions of substrate 1 in p-channel MISFET fabrication areas by a photoresist film 42. Subsequently as shown in FIG. 16, after removal of the photoresist film 42, phosphor is ion-implanted into the surface portions of substrate 1 in the p-channel MISFET fabrication areas while letting the surface portions of substrate 1 in the n-channel MISFET formation areas be covered with a photoresist film 43.

[0102] Next as shown in FIG. 17, after having removed the photoresist film 43, the substrate 1 is thermally processed causing the boron and phosphor dopants as introduced into the substrate 1 to diffuse toward the inside of substrate 1 whereby a semiconductor well region 8 of p type conductivity is formed in the substrate 1 in a respective area for fabrication of an n-channel MISFET while forming an n-type well 9 in each p-channel MISFET formation area of substrate 1.

[0103] Next, two kinds of transistor gate insulating films that are different in thickness from each other are formed on the surface of a respective one of the p-type well 8 and n-well 9, by using a method which follows. Firstly, after having removed portions of the silicon oxide film 10 overlying respective surfaces of p- and n-wells 8, 9 by hydrofluoric acid-based etchant, the substrate 1 is thermally processed at about 800° C. to thereby form a thin silicon oxide film 11 a to its thickness of about 7 to 8 nm on the surface of a respective one of p-and n-wells 8, 9. This silicon oxide film 11 a is for use as part of a gate insulating film with an increased thickness.

[0104] In view of the fact that the nitrogen introduce layer 7 is formed at the surface portion of the silicon oxide film 6 as buried within each element isolation groove 4, it is possible to suppress oxidation of the inner walls of element isolation groove 4 during thermal processing for fabrication of the above-noted silicon oxide film 11 a. Additionally, upon removal of the silicon oxide film 10 using hydrofluoric acid-based etchant, the surface of nitrogen introduce layer 7 also is slightly etched resulting in a further decrease in step-like difference between the surface of substrate 1 in each active region and the surface of nitrogen introduce layer 7; however, as the nitrogen introduce layer 7 will rarely be overetched, it is possible to suppress recessing of edges of element isolation groove 4.

[0105] Next as shown in FIG. 19, a photoresist film 44 is formed covering respective surfaces of the p-type well 8 and n-type well 9 in the thick gate insulating film fabrication area; then, hydrofluoric acid-based etchant is used to remove the silicon oxide film 11 a that has been formed on the surface of a respective one of the p- and n-wells 8-9 in the thin gate insulating film fabrication area.

[0106] Next, after removal of the photoresist film 44 followed by cleaning using hydrofluoric acid-based cleaning liquid of both the surfaces of p- and n-wells 8-9 in the thin gate insulating film fabrication area and surface portions of the silicon oxide film 11 a in the thick gate insulating film formation area, the substrate 1 is thermally processed at about 800° C. Whereby, as shown in FIG. 20, a gate insulating film 12 made of silicon oxide with a thickness of about 5 to 6 nm is formed on the surface of a respective one of the p- and n-wells 8-9 in the thin gate insulating film fabrication area. In addition, as the silicon oxide film 11 a in the thick gate insulating film formation region further increases in its thickness, a gate insulating film 11 made of about 10-nm thick silicon oxide is formed on the surface of a respective one of the p- and n-wells 8-9 in this area.

[0107] As the nitrogen introduce layer 7 is formed at the surface portion of the silicon oxide film 6 buried within each element isolation groove 4, it is possible to suppress oxidation of the inner walls of element isolation groove 4 during thermal processing for formation of the gate insulating films 11-12 stated supra. Additionally, upon removal of the silicon oxide film 11 a by use of hydrofluoric acid-based etchant, the surface of nitrogen introduce layer 7 also is little etched resulting in a further decrease in step-like difference between the surface of substrate 1 in an active region and the surface of nitrogen introduce layer 7; however, the nitrogen introduce layer 7 will rarely be overetched. Thus it becomes possible to suppress recessing at edges of element isolation groove 4.

[0108] Next as shown in FIG. 21, after having deposited a polycrystalline silicon or “poly-silicon” film 14 over the substrate 1 by CVD methods, a portion of the polysilicon film 14 overlying the individual n-type well 9 is covered or coated with a photoresist film 45; then, phosphor is ion-implanted into a portion of polysilicon film 14 overlying each p-type well 8, thereby forming an n-type polysilicon film 14 over the p-well 8.

[0109] Next as shown in FIG. 22, after removal of the photoresist film 45, a portion of the n-type polysilicon film 14 n overlying the n-type well 8 is coated with a photoresist film 46; then, boron is ion-implanted into a portion of polysilicon film 14 overlying the n-type well 9, thus forming a p-type polysilicon film 14 p over n-well 9. The n-type polysilicon film 14 n and p-type polysilicon film 14 p are for later use as MISFET gate electrode material.

[0110] Next as shown in FIG. 23, after having removed the photoresist film 46, the n- and p-type polysilicon films 14 n, 14 p are dry-etched while using as a mask those portions of the photoresist film 47 formed over these films 14 n, 14 p respectively, thereby to form gate electrodes 14A, 14B, 14C and 14D.

[0111] Next, after having removed the photoresist film 47 overlying the gate electrodes 14A-14D by ashing processing, the surface of substrate 1 is cleaned by hydrofluoric acid-based etchant in order to etch away resultant dry-etch and ash residues which are left thereon. Performing dry etching and cleaning using hydrofluoric acid-based etchant of the n-type polysilicon film 14 a and p-type polysilicon film 14 p results in that portions of the gate insulating films 11-12 in areas other than those underlying the gate electrodes 14A-D are thinned while at the same time letting portions of gate insulating films 11-12 at lower part of sidewalls of gate electrodes 14A-D also be isotropically etched away causing creation of undercuts so that gate insulating films 11-12 can decrease in breakdown voltage. To avoid this, the substrate 1 is thermally oxidized for recovery or “regeneration” of gate insulating films 11-12 thinned by the above etch processes (dry etching and wet etching).

[0112] At this time also, the nitrogen introduce layer 7 as formed at the surface portion of silicon oxide film 6 acts to suppress diffusion of oxidizer into silicon oxide film 6 and thus enables suppression of oxidation of inner walls of element isolation grooves 4. In addition, as nitrogen introduce layer 7 is no longer overetched by hydrofluoric acid-based etchant, any unwanted downward retrogradation (recessing) of silicon oxide film 6 at edge portions of element isolation grooves 4 is also suppressed.

[0113] Next as shown in FIG. 24, after having formed lightly-doped n (n⁻) type semiconductor regions 15 in a respective one of the p-type wells 8 at selected surface portions on the opposite sides of gate electrode 14A, 14C and also formed p⁻-type semiconductor regions 16 in each n-type well 9 on the both sides of gate electrode 14B, 14D, a silicon oxide film for use as a sidewall spacer 17 is formed on each sidewall of gate electrodes 14A-14D. The n⁻-type semiconductor regions 15 may be formed by arsenic ion implantation into p-type wells 8 while letting upper part of each n-type well 9 be coated with a photoresist film. Similarly, the p⁻-type semiconductor regions 16 are formed by boron ion implantation into n-wells 9 while letting upper part of each p-well 8 be coated with a photoresist film. The sidewall spacers 17 are formed by applying anisotropic dry etching to a CVD-deposited silicon oxide film over the substrate 1.

[0114] Next as shown in FIG. 25, after having formed heavily-doped n (n⁺) type semiconductor regions 18 (source and drain) in each p-type well 8 at its selected surface portions on the opposite sides of gate electrode 14A, 14C and also p⁺-type semiconductor regions 19 (source, drain) in each n-type well 9 at its surface portions on the both sides of gate electrode 14B, 14D, a cobalt (Co) silicide layer 20 is formed in portions overlying gate electrodes 14A-14D and respective surfaces of the n⁺-type semiconductor regions 18 (source, drain) and p⁺-type semiconductor regions 19 (source, drain), thereby completing n-channel MISFETs Q1 and Q2 and p-channel MISFETS Q3-Q4. The n⁺-type semiconductor regions 18 are formed by arsenic ion implantation into p-wells 8 while letting upper part of each n-well 9 be covered with a photoresist film. In addition, the p⁺-type semiconductor regions 19 are formed by boron ion implantation into n-wells 9 with upper part of each p-well 8 be coated with a photoresist film. The cobalt (Co) silicide layer 20 is formed by a processing including the steps of removing by hydrofluoric acid-based etchant the gate insulating film 11 (or 12) above the surface of a respective one of the n⁺ and p⁺-type semiconductor regions 18-19, depositing a Co film over the substrate 1 by sputter techniques, thermally processing substrate 1, and thereafter removing unreacted portions of the Co film by wet etch methods.

[0115] In accordance with the illustrative embodiment, it is possible to effectively suppress any unwanted oxidation of inner walls of element isolation grooves 4 during several thermal oxidation process steps to be carried out along the way of MISFET fabrication process, which in turn makes it possible to reduce compressive force being applied to certain portions of the substrate 1 in its active regions due to oxidation of the inner walls of element isolation grooves 4. Whereby, crystal defects and/or dislocations that are induced by the compressive force application is suppressed, resulting in suppression of degradation of device characteristics such as a current leakage increase; thus, the resultant CMOS devices may improve in reliability and production yields.

[0116] In addition, according to this embodiment, it is also possible to avoid the problem as to downward recessing of the silicon oxide film 6 at edges of element isolation grooves 4 otherwise occurring due to several hydrofluoric acid cleaning processes to be done in the fabrication of MISFETs. This in turn makes it possible to suppress creation of parasitic MOS components as induced by the recessing phenomenon, resulting in the MISFET characteristics being made stable; thus, the CMOS devices may improve in reliability and manufacturing yields.

[0117] Embodiment 2:

[0118] A fabrication method in accordance with a second embodiment of the invention will be set forth in conjunction with FIGS. 26 to 30 below. Initially as shown in FIG. 26, after having formed grooves 4 a in a substrate 1 by dry etching with a silicon nitride film 3 as a mask, a silicon oxide film 6 deposited over substrate 1 is planarized by CMP techniques. The process steps up to here are similar to those of the aforesaid embodiment shown in FIGS. 1 to 7.

[0119] Next as shown in FIG. 27, after having wet-etched the surface of a portion of the silicon oxide film 6 residing within a groove 4 a to thereby retrograde it downwardly, a silicon nitride film 30 over substrate 1 is deposited by CVD techniques. The silicon nitride film 30 is deposited to a thickness sufficiently less than a diameter of groove 4 a.

[0120] As shown herein, depositing the silicon nitride film 30 over the substrate 1 in the state that the silicon oxide film 6's surface is downwardly retrograded so that it is lower in level than the silicon nitride film 3's surface results in film thickness (thickness at right angles to a principal surface of substrate 1) of silicon nitride film 30 at sidewall portions of silicon nitride film 3—namely, at the periphery of groove 4 a—becoming thicker than the remaining regions.

[0121] Next as shown in FIG. 28, nitrogen is doped by ion implantation into an entire surface area of the substrate 1. At this time, the acceleration energy of nitrogen ions is appropriately controlled in such a way as to prevent arrival of nitrogen ions at the substrate 1's surface of active regions after penetration of the silicon nitride film 3. With such acceleration energy control, nitrogen is hardly introduced into portions of the silicon oxide film 6 underlying the silicon nitride film 3 because of the fact that peripheral portions of groove 4 a are coated with the silicon nitride film 3 of a relatively large thickness.

[0122] Next as shown in FIG. 29, the substrate 1 is applied thermal processing for activation of nitrogen doped, thereby forming a nitrogen introduce layer 7 at a surface portion of the silicon oxide film 6. This nitrogen introduce layer 7 is high in nitrogen concentration at a central portion of groove 4 a and becomes lower at portions adjacent to the active regions in substrate 1.

[0123] Next as shown in FIG. 30, hot hydrofluoric acid is used to remove both the silicon nitride film 30 covering the entire surface of the substrate 1 and the silicon nitride film 3 covering the active regions in substrate 1 to thereby form a circuit element isolation groove 4 in each element isolation area of substrate 1. Its following process steps are similar in principle to those of the embodiment 1 stated supra.

[0124] In accordance with the embodiment 2, specifically designing the nitrogen concentration of the nitrogen introduce layer 7 so that it stays low at adjacent portions to the active regions in the substrate 1 makes it possible to suppress diffusion of nitrogen residing in nitrogen introduce layer 7 toward portions of substrate 1 at active region edges during thermal processing to be later effectuated, which in turn minimizes risks of breakdown voltage dropdown of gate insulating films at locations adjacent to active region edges and/or variation in threshold voltage of MISFETs.

[0125] Embodiment 3:

[0126] A fabrication method in accordance with a third embodiment of the invention will be described with reference to FIGS. 31 to 33 below. Firstly as shown in FIG. 31, after having formed grooves 4 a in a substrate 1 by dry etching with a silicon nitride film 3 as a mask, thermal processing is applied to substrate 1 to thereby form a thin silicon oxide film 5 on inner walls of each groove 4 a. The process steps up to here are similar in principle to those of the previous embodiment shown in FIGS. 1 to 4.

[0127] Next, as shown in FIG. 32, CVD technique is used to deposit a thin silicon nitride film 31 over the substrate 1 to a thickness of about 5 to 10 nm. Subsequently as shown in FIG. 33, after having buried a silicon oxide film 6 within the groove 4 a by use of a method similar to that of the embodiment 1 and then planarized its surface, fabricate a nitrogen introduce layer 7 at a surface portion of silicon oxide film 6. Its following process is similar to that of the embodiment 1 stated supra.

[0128] In this way, with the embodiment 3, the silicon nitride film 31 is formed along inner walls of element isolation groove 4 while forming the nitrogen introduce layer 7 at the surface portion of silicon oxide film 6 as buried within element isolation groove 4; thus, it is possible to effectively suppress oxidation of the inner walls of element isolation groove 4 during thermal oxidation processes.

[0129] Embodiment 4:

[0130] A fabrication method in accordance with a fourth embodiment will next be explained using FIGS. 34 to 36. First, as shown in FIG. 34, after having formed grooves 4 a in a substrate 1 by dry etch techniques with a silicon nitride film 3 as a mask, substrate 1 is thermally processed to thereby form a thin silicon oxide film 5 on inner surfaces—namely a bottom surface and sidewalls—of each groove 4 a. The process up to here is similar to that of the previous embodiment shown in FIGS. 1 to 4.

[0131] Next as shown in FIG. 35, a nitrogen containing layer 32 is fabricated on the bottom surface and sidewalls of groove 4 a. This nitrogen containing layer 32 is manufacturable by a method having the steps of thermally processing the substrate 1 in an environment of a mixture of NO and N₂ gases—here, 10% of NO contained—at temperatures ranging for example from 900 to 950° C., and then forcing segregation of nitrogen into the bottom face and sidewalls of groove 4 a. Alternatively the nitrogen containing layer 32 may be formed through thermal processing of substrate 1 in a N₂O gaseous atmosphere at 1000 to 1050° C.

[0132] Next as shown in FIG. 36, after having buried a silicon oxide film 6 within the groove 4 a by using a similar method to that of the previous embodiment 1 followed by planarization of the surface thereof, a nitrogen introduce layer 7 is formed at a surface portion of silicon oxide film 6. Although the nitrogen introduce layer 7 is capable of being formed by the above-noted ion implantation technique, the same results are obtainable by another method including the step of exposing the substrate 1 to a high-density nitrogen plasma atmosphere. Still another approach is to employ a method having the steps of performing this nitrogen plasma processing and thereafter thermally processing substrate 1 for causing outdiffusion of nitrogen within silicon oxide film 6 to thereby adjust a depth of nitrogen introduce layer 7 and/or a profile of nitrogen doped.

[0133] In this way, with the embodiment 4, the nitrogen containing layer 32 is formed on the bottom and sidewalls of element isolation groove 4 while simultaneously forming the nitrogen introduce layer 7 at the surface portion of silicon oxide film 6 as buried within element isolation groove 4. This nitrogen containing layer 32 is expected to function along with nitrogen introduce layer 7 as a barrier layer for suppression of diffusion of an oxidizer used; thus, it becomes possible to effectively suppress oxidation of the inner surfaces of element isolation groove 4 during thermal oxidation processes.

[0134] Embodiment 5:

[0135] A fabrication method in accordance with a fifth embodiment will be discussed using FIGS. 37 to 41. Firstly as shown in FIG. 37, after having formed grooves 4 a in a substrate 1 by dry etching with silicon nitride film 3 as a mask, CMP processing is performed to planarize a silicon oxide film 6 deposited over substrate 1. The process steps up to here is the same as those of the previous embodiment 1 shown in FIGS. 1 to 7.

[0136] Next as shown in FIG. 38, a selected portion of the substrate 1 in an area for formation of a thick gate insulating film(s) is coated with a photoresist film 48; then, doped by ion implantation nitrogen into substrate 1 in another area for fabrication of a thin gate insulating film(s). At this time, the acceleration energy of nitrogen ions is controlled to ensure that nitrogen ions pass through silicon nitride film 3 to reach surface portions of substrate 1 in active regions thereof.

[0137] Next as shown in FIG. 39, after having removed the photoresist film 48, the substrate 1 is thermally processed for activation of the doped nitrogen to thereby form a nitrogen introduce layer 7 at a surface portion of silicon oxide film 6 as buried within groove 4 a in the thin gate insulating film formation area.

[0138] Next, after having performed necessary processing including removal of silicon nitride film 3 and formation of wells (p- and n-type wells 8, 9) in accordance with the process steps of the embodiment 1 as shown in FIGS. 12 to 17, the substrate 1 is thermally processed at about 800° C. to form a silicon oxide film 33 for use as a gate insulating film with a thickness of about 10 nm on the surface of a respective one of the p- and n-type wells 8-9 in the thick gate insulating film formation area as shown in FIG. 40. Although at this time respective surfaces of p- and n-type wells 8-9 in the thin gate insulating film formation area are also oxidized, oxidation rates stay lower due to the fact that these surfaces have been doped with nitrogen. The result is that a gate insulating film 34 made of silicon oxide (with a thickness of 5 to 6 nm for example) which is thinner than the above-stated gate insulating film 33 is formed on this surface.

[0139] Thereafter as shown in FIG. 41, n-channel MISFETs Q1-Q2 and p-channel MISFETs Q3-4 are formed in accordance with the processes of the previous embodiment shown in FIGS. 21 to 25.

[0140] In this way, with the embodiment 5, two types of gate insulating films 33 and 34 different in thickness from each other are formed substantially simultaneously at a single thermal oxidation step; thus, it is possible to shorten the CMOS device fabrication process. Additionally, the requisite number of thermal oxidation and hydrofluoric acid cleaning steps is reduced, thus enabling suppression of oxidation of inner walls of element isolation grooves 4 and/or recessing of the substrate at active region edges without requiring formation of nitrogen introduce layer 7 at element isolation grooves 4 in the thick gate insulating film fabrication area.

[0141] Optionally it is also possible to form the two types of thickness different gate insulating films 33-34 simultaneously at a single thermal oxidation process step by a method which includes the steps of covering the substrate 1 with a photoresist film in the thick gate insulating film fabrication area after having formed the p-and n-type wells 8-9 in substrate 1 in accordance with the process of the previous embodiment 1 shown in FIGS. 1 to 17 by way of example and then selectively doping nitrogen by ion implantation into substrate 1 in the thin gate insulating film formation area.

[0142] Embodiment 6:

[0143] A sixth embodiment will be discussed using FIGS. 42 to 48. Initially as shown in FIG. 42, after having formed grooves 4 a in substrate 1 by dry etch techniques with silicon nitride film 3 as a mask, a silicon oxide film 6 deposited over substrate 1 is planarized by CMP methods. Then, surface portions of silicon oxide film 6 residing within the grooves 4 a is wet-etched by hydrofluoric acid-based etchant, causing the silicon oxide film 6's surface to little retrograde downwardly. The process steps up to here are the same as those of the above embodiment 1 shown in FIGS. 1 to 9.

[0144] Next, after having ion-implanted nitrogen into an entire surface of the substrate 1 as shown in FIG. 43, silicon (Si) is ion-implanted into the entire surface of substrate 1 as shown in FIG. 44. Note here that these nitrogen and silicon ion implantation steps are freely determinable in order of sequence on a case-by-case basis.

[0145] Next as shown in FIG. 45, the substrate 1 is thermally processed, forming a nitrogen introduce layer 7 at a surface portion of silicon oxide film 6. Performing this thermal processing results in creation of silicon oxynitride (SiON) at the surface portion of silicon oxide film 6 by replacement of part of Si—O bonds making up silicon oxide film 6 with Si—N bonds and simultaneously production of Si—N bonds through reaction of the nitrogen and silicon as introduced by ion implantation. Its following process steps are the same as those of the prescribed Embodiment 1.

[0146] In this way, with the embodiment 6, both nitrogen and silicon are ion-implanted into the surface portion of each portion of the silicon oxide film 6 as buried in the individual groove 4 a and are then forced to react together by thermal processing applied thereto. With such process scheme, it is possible to readily fabricate the intended nitrogen introduce layer 7 high in Si—N bond concentration when compared to the case where nitrogen alone is doped into silicon oxide film 6 because of the fact that the coupling energy of nitrogen and silicon is lower than an energy for replacement of Si—O bonds with Si—N bonds; thus, it becomes possible to more effectively suppress oxidation of inner walls of grooves 4 a and recessing of active region edge portions.

[0147] Note here that the nitrogen introduction or doping may alternatively be achievable by use of the previously stated nitrogen plasma method other than ion implantation methods. More specifically the nitrogen introduce layer 7 may be formed by a process having the steps of exposing the substrate 1 to a nitrogen plasma atmosphere after or prior to execution of ion implantation of silicon into an entire surface of substrate 1, and thereafter applying thermal processing to substrate 1.

[0148] Still alternatively the nitrogen introduce layer 7 may be fabricated by utilization of a thermal oxidation process after introduction of silicon and nitride into silicon oxide film 6. An example of this approach is as follows: after having ion-implanted silicon and nitride into the entire surface of substrate 1 as shown in FIG. 46, hot phosphoric acid is used to remove silicon nitride film 3 as shown in FIG. 47, and thereafter substrate 1 is thermally processed as shown in FIG. 48 to thereby form nitrogen introduce layer 7 at the surface portion of silicon oxide film 6. In this case, it is also possible to reduce the requisite nitrogen doping amount and/or eliminate nitrogen doping process per se since the oxidizer in an atmosphere diffusing into silicon oxide film 6 during thermal processing is consumed through reaction with silicon.

[0149] Furthermore, the method as has been discussed in conjunction with the embodiment 2 is employable to lower the nitrogen concentration of nitrogen introduce layer 7 at portions adjacent to the active regions in the substrate 1 with or without use of the method as stated in relation to any one of the embodiments 3 and 4 stated supra to form either the silicon nitride film 31 or the nitrogen containing layer 32 along inner walls of element isolation grooves 4.

[0150] Although the invention made by the inventor(s) as named herein has been described in detail based on several embodiments thereof, this invention should not be limited only to these embodiments and may be modifiable and alterable in a variety of ways without departing the spirit and scope of the invention.

[0151] For example, a plurality of nitrogen introduce layers may be formed within a silicon oxide film by a process having the steps of performing ion implantation while varying the acceleration energy during ion implantation of nitrogen into the silicon oxide film within an element isolation groove(s) and thereafter performing thermal processing.

[0152] The thermal processing for formation of the nitrogen introduce layer will not necessarily be performed immediately after introduction of nitrogen into the silicon oxide film within the element isolation groove(s) and may alternatively be done by utilizing thermal processing to be later effectuated.

[0153] Although the embodiments stated above are specifically directed to CMOS device applications, the principles of the invention as disclosed and claimed herein may also be applied to other types of MOS devices having a plurality of thickness different gate insulating films, including but not limited to flash memories, memory-embedded logic devices, and the like.

[0154] A brief explanation of effects and advantages obtainable by the representative ones of the invention as disclosed herein is as follows.

[0155] By introducing nitrogen into a surface portion of a silicon oxide film as buried in an element isolation groove, it becomes possible to effectively suppress unwanted oxidation of inner walls of the element isolation groove otherwise occurring during thermal oxidation processes.

[0156] Introducing nitrogen into the surface portion of a silicon oxide film as buried within an element isolation groove makes it possible to effectively suppress recessing of the element isolation groove occurring at hydrofluoric acid cleaning steps. 

What is claimed is:
 1. A semiconductor integrated circuit device comprising: (a) a semiconductor substrate having at its principal surface a first area and a second area planarly surrounded by the first area; (b) a first insulative film formed over the principal surface of the semiconductor substrate in the second area; (c) a conductor piece formed over the first insulative film; (d) a semiconductor region formed at the semiconductor substrate principal surface on both sides of the conductor piece; and (e) an oxide film being formed at the semiconductor substrate principal surface in the first area and having a film thickness greater than that of the first insulative film, wherein nitrogen is introduced at a surface portion of the oxide film.
 2. The semiconductor integrated circuit device according to claim 1, wherein the nitrogen is introduced up to a position lower than the semiconductor substrate principal surface in the second area.
 3. The semiconductor integrated circuit device according to claim 1, wherein the oxide film has a portion in which the nitrogen is not included.
 4. The semiconductor integrated circuit device according to claim 1, wherein a concentration of the nitrogen as introduced into the oxide film is high at a central portion of the first area and is relatively low at a portion adjacent to the second area.
 5. The semiconductor integrated circuit device according to claim 3, wherein a layer containing nitrogen therein is present at an interface between the oxide film at the portion in which the nitrogen is not included and the second area.
 6. The semiconductor integrated circuit device according to claim 3, wherein a nitride film is present between the oxide film at the portion in which the nitrogen is not included and the second area.
 7. A semiconductor integrated circuit device comprising: (a) a semiconductor substrate having at its principal surface a first area, a second area and a third area, the second and third areas being planarly surrounded by the first area; (b) a first insulative film formed above the semiconductor substrate principal surface in the second area; (c) a second insulative film being formed over the semiconductor substrate principal surface in the third area and being less in film thickness than the first insulative film; (d) a first conductor piece formed over the first insulative film; (e) a second conductor piece formed over the second insulative film; (f) a first semiconductor region formed at the semiconductor substrate principal surface on both sides of the first conductor piece in the second area; (g) a second semiconductor region formed at the semiconductor substrate principal surface on both sides of the second conductor piece in the third area; and (h) an oxide film being formed at the semiconductor substrate principal surface in the first area and being greater in film thickness than the first and second insulative films, wherein nitrogen is introduced at a surface portion of the oxide film.
 8. The semiconductor integrated circuit device according to claim 7, wherein the nitrogen as introduced in the oxide film is such that a concentration at a portion adjacent to the second area is less than that at a portion adjacent to the third area.
 9. The semiconductor integrated circuit device according to claim 7, wherein the oxide film with the nitrogen introduced thereinto is such that a width on a side adjacent to the second area is greater than that on another side adjacent to the third area.
 10. The semiconductor integrated circuit device according to claim 7, wherein the nitrogen is introduced up to a position lower than the semiconductor substrate principal surface in the second and third areas.
 11. The semiconductor integrated circuit device according to claim 7, wherein the oxide film has a portion in which the nitrogen is not contained.
 12. The semiconductor integrated circuit device according to claim 7, wherein a concentration of the nitrogen as introduced in the oxide film is high at a central portion of the first area and is relatively low at a portion adjacent to the second and third areas.
 13. The semiconductor integrated circuit device according to claim 11, wherein a nitrogen containing layer is present at an interface between the oxide film at the portion in which the nitrogen is not included and the second and third areas.
 14. The semiconductor integrated circuit device according to claim 11, wherein a nitride film is present between the oxide film at the portion in which the nitrogen is not included and the second and third areas.
 15. A method of fabricating a semiconductor integrated circuit device, comprising: (a) exposing a first area of a semiconductor substrate principal surface to thereby selectively form a first insulative film at the semiconductor substrate principal surface in a second area as surrounded by the first area; (b) forming a groove in the semiconductor substrate surface in the first area; (c) forming a second insulative film within the groove and over the first insulative film in such a way as to bury the groove; (d) applying polishing to the second insulative film to thereby permit the second insulative film to selectively remain within the groove; (e) introducing nitrogen into the second insulative film within the groove; (f) removing the first insulative film of the second area; (g) forming a third insulative film at the semiconductor substrate surface in the second area; (h) forming a conductor piece over the third insulative film; and (i) introducing in the second area an impurity into the semiconductor substrate surface on both sides of the conductor piece.
 16. The method of fabricating a semiconductor integrated circuit device according to claim 15, wherein introduction of the nitrogen is done by an ion implantation method.
 17. The method of fabricating a semiconductor integrated circuit device according to claim 16, wherein thermal processing is applied to the semiconductor substrate after introduction of the nitrogen.
 18. The method of fabricating a semiconductor integrated circuit device according to claim 15, wherein introduction of the nitrogen is done in a plasma atmosphere.
 19. The method of fabricating a semiconductor integrated circuit device according to claim 18, including, between the step (d) and the step (e), the step of applying etching to the second insulative film being left within the groove.
 20. A method of fabricating a semiconductor integrated circuit device, comprising: (a) exposing a first area of a semiconductor substrate principal surface to thereby selectively form a silicon nitride film at the semiconductor substrate principal surface in a second area; (b) forming a groove in the semiconductor substrate surface in the first area; (c) forming a first silicon oxide film within the groove to bury the groove; (d) introducing nitrogen into the first silicon oxide film within the groove; (e) removing the silicon nitride film in the second area; (f) forming a second silicon oxide film at the semiconductor substrate surface in the second area; (g) forming a conductor piece over the second silicon oxide film; and (h) introducing an impurity into the semiconductor substrate surface on both sides of the conductor piece in the second area.
 21. A method of fabricating a semiconductor integrated circuit device, comprising: (a) exposing a first area of a semiconductor substrate principal surface to thereby selectively form a first insulative film at the semiconductor substrate principal surface in a second area as surrounded by the first area; (b) forming a groove in the semiconductor substrate surface in the first area; (c) forming a second insulative film within the groove and over the first insulative film to bury the groove; (d) applying polishing to the second insulative film to thereby permit the second insulative film to selectively remain within the groove; (e) introducing nitrogen by an ion implantation method into the second insulative film within the groove; (f) removing the first insulative film in the second area; (g) forming a third insulative film at the semiconductor substrate surface in the second area with the first insulative film removed away; (h) forming a conductor piece over the third insulative film; and (i) introducing in the second area an impurity into the semiconductor substrate surface on both sides of the conductor piece.
 22. The method of fabricating a semiconductor integrated circuit device according to claim 21, wherein thermal processing is applied to the semiconductor substrate after having introduced the nitrogen.
 23. A method of fabricating a semiconductor integrated circuit device, comprising: (a) exposing a first area of a semiconductor substrate principal surface to thereby selectively form a first insulative film at the semiconductor substrate principal surface in a second area and a third area, the second and third areas being surrounded by the first area; (b) forming a groove in the semiconductor substrate surface of the first area; (c) forming a second insulative film within the groove and over the first insulative film to bury the groove; (d) polishing the second insulative film to permit the second insulative film to selectively remain within the groove; (e) introducing nitrogen into the second insulative film in the first area; (f) removing the first insulative film in the second and third areas; (g) forming a third insulative film at the semiconductor substrate surface in the second and third areas; (h) selectively removing the third insulative film in the third area while letting the third insulative film remain in the second area; (i) applying thermal processing to the semiconductor substrate in an oxygen atmosphere to thereby form a fourth insulative film at the semiconductor substrate surface of the second area while forming at the semiconductor substrate surface of the third area a fifth insulative film less in thickness than the fourth insulative film; (j) forming a first conductor piece over the fourth insulative film and a second conductive piece over the fifth insulative film; and (k) introducing in the second area an impurity into the semiconductor substrate surface on both sides of the first conductor piece while introducing in the third area an impurity into the semiconductor substrate surface on both sides of the second conductor piece.
 24. A method of fabricating a semiconductor integrated circuit device, comprising: (a) exposing a first area of a semiconductor substrate principal surface to thereby selectively form a first insulative film at the semiconductor substrate principal surface in a second area and a third area, the second and third areas being surrounded by the first area; (b) forming a groove in the semiconductor substrate surface of the first area; (c) forming a second insulative film within the groove and over the first insulative film to bury the groove; (d) polishing the second insulative film to permit the second insulative film to selectively remain within the groove; (e) introducing nitrogen into the second insulative film within the groove; (f) forming a third insulative film at the semiconductor substrate surface in the second third areas; (g) selectively removing the third insulative film in the third area while letting the third insulative film be left in the second area; (h) applying thermal processing to the semiconductor substrate in an oxygen atmosphere to thereby form a fourth insulative film at the semiconductor substrate surface of the second area while forming at the semiconductor substrate surface of the third area a fifth insulative film less in thickness than the fourth insulative film; (i) forming a first conductor piece over the fourth insulative film and a second conductive piece over the fifth insulative film; and (j) introducing in the second area an impurity into the semiconductor substrate surface on both sides of the first conductor piece while introducing in the third area an impurity into the semiconductor substrate surface on both sides of the second conductor piece.
 25. A method of fabricating a semiconductor integrated circuit device, comprising: (a) exposing a first area of a semiconductor substrate principal surface to thereby selectively form a first insulative film at the semiconductor substrate principal surface in a second area and a third area, the second and third areas being surrounded by the first area; (b) forming a groove in the semiconductor substrate surface of the first area; (c) forming a second insulative film within the groove and over the first insulative film to bury the groove; (d) polishing the second insulative film to permit the second insulative film to selectively remain within the groove; (e) introducing nitrogen into the semiconductor substrate surface in the third area; (f) removing the first insulative film in the second area and the third area; (g) thermally processing the semiconductor substrate surface with the first insulative film removed therefrom in an oxygen atmosphere to thereby form a third insulative film at the semiconductor substrate surface in the second area while forming at the semiconductor substrate surface of the third area a fourth insulative film being less in thickness than the third insulative film; (h) forming a first conductor piece over the third insulative film and a second conductive piece over the fourth insulative film; and (i) introducing in the second area an impurity into the semiconductor substrate surface on both sides of the first conductor piece while introducing in the third area an impurity into the semiconductor substrate surface on both sides of the second conductor piece.
 26. A method of fabricating a semiconductor integrated circuit device, comprising: (a) exposing a first area of a semiconductor substrate principal surface to thereby selectively form a first insulative film at the semiconductor substrate principal surface in a second area and a third area, the second and third areas being surrounded by the first area; (b) forming a groove in the semiconductor substrate surface of the first area; (c) forming a second insulative film within the groove and over the first insulative film to bury the groove; (d) polishing the second insulative film to permit the second insulative film to selectively remain within the groove; (e) introducing nitrogen into the second insulative film in the first area; (f) introducing nitrogen into the semiconductor substrate surface of the third area; (g) removing the first insulative film of the second and third areas; (h) thermally processing the semiconductor substrate in an oxygen atmosphere to thereby form a third insulative film at the semiconductor substrate surface of the second area while forming at the semiconductor substrate surface of the third area a fourth insulative film being less in thickness than the third insulative film; (i) forming a first conductor piece over the third insulative film and a second conductive piece over the fourth insulative film; and (j) introducing in the second area an impurity into the semiconductor substrate surface on both sides of the first conductor piece while introducing in the third area an impurity into the semiconductor substrate surface on both sides of the second conductor piece.
 27. A method of fabricating a semiconductor integrated circuit device, comprising: (a) exposing a first area of a semiconductor substrate principal surface to thereby selectively form a first insulative film at the semiconductor substrate principal surface of a second area as surrounded by the first area; (b) forming a groove in the semiconductor substrate surface of the first area; (c) forming a second insulative film within the groove and over the first insulative film to bury the groove; (d) polishing the second insulative film to cause the second insulative film to selectively remain within the groove; (e) etching the second insulative film thereby letting a surface of the second insulative film be lower than a surface of the first insulative film; (f) forming a third insulative film over the first and second insulative films; (g) introducing nitrogen into the second insulative film in the first area through the third insulative film; (h) removing the first and third insulative films of the second area; (i) forming a fourth insulative film at the semiconductor substrate surface of the second area; (j) forming a conductor piece over the fourth insulative film of the second area; and (k) introducing in the second area an impurity into the semiconductor substrate surface on both sides of the conductor piece.
 28. The method of fabricating a semiconductor integrated circuit device according to claim 27, wherein the nitrogen is prevented from being introduced into a lower portion of the third insulative film as formed along a side wall of the first insulative film.
 29. A method of fabricating a semiconductor integrated circuit device, comprising: (a) exposing a first area of a semiconductor substrate principal surface to thereby selectively form a first insulative film at the semiconductor substrate principal surface of a second area surrounded by the first area; (b) forming a groove in the semiconductor substrate surface of the first area; (c) thermally processing the semiconductor substrate in a nitrogen-containing atmosphere; (d) forming a second insulative film within the groove and over the first insulative film to bury the groove; (e) polishing the second insulative film to permit the second insulative film to be left selectively in the first area; (f) introducing nitrogen into the second insulative film of the first area; (g) forming a third insulative film at the semiconductor substrate surface of the second area with the first insulative film removed away therefrom; (h) forming a conductor piece over the third insulative film; and (i) introducing in the second area an impurity into the semiconductor substrate surface on both sides of the conductor piece.
 30. The method of fabricating a semiconductor integrated circuit device according to claim 29, including between the step (e) and the step (f): (j) etching the second insulative film to thereby let a surface of the second insulative film be lower than a surface of the first insulative film; and (k) forming a fourth insulative film over the first and second insulative films, and in that the nitrogen is introduced into the second insulative film through the fourth insulative film.
 31. A method of fabricating a semiconductor integrated circuit device, comprising: (a) exposing a first area of a semiconductor substrate principal surface to selectively form a first nitride film over the semiconductor substrate principal surface of a second area surrounded by the first area; (b) forming a groove in the semiconductor substrate surface of the first area; (c) forming a first oxide film at a surface of the groove; (d) forming a second nitride film over the first oxide film; (e) forming a second oxide film within the groove and over the first nitride film to bury the groove; (f) polishing the second oxide film to cause the second oxide film to be selectively left in the first area; (g) introducing nitrogen into the second oxide film of the first area; (h) forming a third oxide film at the semiconductor substrate surface of the second area with the first and second nitride films removed away therefrom; (i) forming a conductor piece over the third oxide film; and (j) introducing in the second area an impurity into the semiconductor substrate surface on both sides of the conductor piece.
 32. The method of fabricating a semiconductor integrated circuit device according to claim 31, including between the step (e) and step (f): (k) etching the second oxide film to let a surface of the second oxide film be lower than a surface of the first nitride film; and (1) forming a insulative film over the first and second nitride films and over the second oxide film, and in that the nitrogen is introduced into the second oxide film via through the insulative film.
 33. A method of fabricating a semiconductor integrated circuit device, comprising: (a) exposing a first area of a semiconductor substrate principal surface to selectively form a first insulative film at the semiconductor substrate principal surface of a second area as surrounded by the first area; (b) forming a groove in the semiconductor substrate surface of the first area; (c) forming a second insulative film within the groove and over the first insulative film to bury the groove; (d) polishing the second insulative film to let the second insulative film selectively remain within the groove; (e) doping nitrogen and silicon into the second insulative film within the groove; (f) removing the first insulative film of the second area; (g) forming a third insulative film at the semiconductor substrate surface of the second area; (h) forming a conductor piece over the third insulative film; and (i) doping in the second area an impurity into the semiconductor substrate surface on both sides of the conductor piece.
 34. The method of fabricating a semiconductor integrated circuit device according to claim 33, wherein the nitrogen is doped by an ion implantation method or in a plasma atmosphere whereas the silicon is doped by ion implantation.
 35. The method of fabricating a semiconductor integrated circuit device according to claim 33, wherein the semiconductor substrate is thermally processed after having doped the nitrogen and silicon. 